The present invention relates to helicopter automatic piloting equipment.
To stabilize and direct his craft, a helicopter pilot manually actuates piloting means (cyclic stick, collective pitch and pedals) to act on the helicopter piloting axes (main rotor or tail rotor). A lateral or longitudinal displacement of the cyclic stick makes it possible to act respectively on the lateral or longitudinal axis of the helicopter by modifying the incidence of the blades of the main rotor. The collective pitch makes it possible to adapt the engine power to the flight conditions by modifying the incidence of the blades of the main rotor. The pedals make it possible to orient the nose of the helicopter by modifying the incidence of the blades of the tail rotor. The motion of the piloting means is transmitted to the piloting axes by means of mechanical transmission chains which consist of various mechanical relays. The assembly consisting of a piloting means and an associated mechanical transmission chain constitutes a flight control linked to the piloting axis considered. The end of the flight control in contact with the associated piloting means is called the output end of the flight control.
Like any dynamic mechanical system, flight controls are affected by mechanical play which implies that the motion of the piloting means is not fully retransmitted to the piloting axes.
Helicopters are often equipped with automatic piloting equipment which acts on the flight controls, under the authority of the pilot, with a view to fulfilling two main missions: a first mission of pilot assistance, and a second mission of automatic piloting.
When it assists the pilot in the manual command of his helicopter, the automatic piloting equipment makes it possible on the one hand to dampen the movements of the machine so as to facilitate control thereof by the pilot, and on the other hand to maintain the current flight configuration (lateral and longitudinal trims, and heading) thus allowing the pilot to momentarily release the piloting means without placing himself in a flight configuration that would be dangerous.
When it is in automatic piloting mode, the automatic piloting equipment makes it possible to slave one or more flight parameters (altitude, vertical speed, longitudinal speed, lateral speed, heading, navigation, etc.) to one or more instruction values chosen beforehand by the pilot.
To act on a flight control, the automatic piloting equipment employs rams, called “series rams” because they are in series with the flight controls. “Series rams” are mechanical actuators which comprise a body and an output axis, they are in general of the endless screw/nut type and they have a short stroke and a small response time. They transform an electrical control into a translational motion of their output axis with respect to their body.
“Series rams” are said to be “mechanically irreversible”, that is to say they deform only when an electrical control is applied to them. In particular when the automatic piloting equipment is not operational, the “series rams” have no effect on the command of the helicopter.
The displacement of the output axis of the “series ram” is slaved in position on the basis of the position information delivered by a position sensor which indicates the position of the output axis of the “series ram” with respect to its body.
Like any dynamic mechanical system, “series rams” are affected by mechanical play. On account of the slaving to the position of the output axis of the ram, this ram play is compensated for at each change of direction by a more significant travel of the motorization of the ram. During the time necessary for the play to be taken up, the position of the output axis of the ram with respect to the body of the ram is fixed. The output axis is slaved to the requested position only when the play is compensated.
The mechanical play of the flight controls and of the “series rams”, even if reduced, increases the difficulty of piloting a helicopter in particular under turbulent flight conditions.
Although the “series rams” are integrated with the flight controls of the helicopter and insofar as their impact is different, a distinction is drawn hereinafter between on the one hand the play of the “series rams” and on the other hand the play of the flight controls. These mechanical plays produce three different effects:                a delay induced by the time necessary for the “series ram” to take up its play after each change of direction of displacement. Insofar as the slaving is done with regard to the position of the “series ram”, there is no loss of effectiveness of the control at the “series ram” level.        a delay induced by the time necessary for the “series ram” to traverse the play of the flight controls after each change of direction of displacement.        a loss of amplitude of the control actually transmitted due to the fact that the displacement of the flight control is not slaved in position. This loss of amplitude corresponds to a loss of effectiveness of the control.        
These three effects together cause a very sharp degradation in the performance of the automatic piloting equipment, by inducing for example a sustained oscillation of its attitudes. These effects are perceptible both in assisted piloting mode and in automatic piloting mode, since in both cases, the commands aimed at acting on the helicopter piloting axes are transmitted by way of the flight controls.
A solution for alleviating this problem consists in reducing the plays by attending to the mechanical realization of the helicopter flight controls. This approach is rapidly halted by the technical difficulties and costs.